This graphic shows why a Labour Party split would be total electoral suicide

There is serious talk of a Labour Party split in the coming
months. If, as expected, Jeremy Corbyn wins the upcoming
leadership election next month, a group of anti-Corbyn MPs are
willing to split off and form a centre-left parliamentary party.
Business Insider's Jim Edwards reported this in detail on Tuesday.

Labour's biggest donor, the millionaire John Mills, described the
downsides of a split as "huge" and it is very hard to disagree
when you take a look at this new YouGov data. The graphic below
illustrates how a Labour split would slash the party's already
dwindling support with the electorate as a whole by a significant
amount, which in a First Past The Post system, would translate
into a total electoral disaster.

YouGov

The Conservative party currently has a 10-point lead over Labour
when it comes to voter intention, according to YouGov —
Conservatives 39% / Labour 29%. However, if Labour's centrists
were to split off and leave Corbyn and his team in charge of
Labour in its official form, the party would receive just 21% of
the vote.

Similarly, if Corbynistas were to defect and leave more moderate
MPs in control of the parliamentary Labour Party, just 19% of
people would vote for it in the coming election. In both cases,
the split-off parties would receive even smaller portions of the
national vote.

You do not have to be a political scientist to take one look at
these figures and realise how terrible the implications would be.
Labour is already performing awfully in the
polls at a stage in the electoral cycle when they
should be leading the Tories — never mind trailing by
10-12 points. Even as a single party, a heavy defeat is looking
the most likely outcome for Labour in the 2020 general election.

Labour in a post-split form would be an electoral car crash. It
would likely perform only a bit better than UKIP. It would all
but guarantee a Conservative victory in 2020, and probably
decades of Tory rule. As neatly summed up by YouGov's Anthony
Wells:

"The two sides of Labour standing as separate parties win a
little more support in total than Labour would standing as a
single party. In a proportional electoral system, this would be
better for them… but in our current First Past the Post electoral
system splitting the Labour vote this way would be disastrous for
them unless their votes were strongly geographically
concentrated."

It is worth nothing that hypothetical polling isn't renowned for
its accuracy and these figures must be approached with caution.
But it looks like the worst is yet to come for the crisis-riddled
party.